A few years ago I left the company I was was working for. We had an internal doc wiki that hardly anyone used. I was one of the ones who did and I would document code changes and things like how to setup a dev environment and to list known gotchas. During my final week when I was doing code handover I sent an email around the company pointing out that the wiki would answer most of the questions they might have about…
you actually helped them? Hang up, charge your emergency hour consultant rates, and tell them you're available once they sign the agreement.